As various electronic apparatuses such as mobile phones, notebook personal computers, and digital cameras become widespread, research and development of various power storage devices as cordless power sources for these electronic apparatuses have been actively performed. Of these power storage devices, an electric double layer capacitor is widely used for applications such as a backup power source for a personal computer memory, an auxiliary power source for a hybrid car, and the like, because it can be charged and discharged at a high speed, and its performance is less deteriorated even when it is repeatedly charged and discharged.
The electric double layer capacitor utilizes the fact that an ultra-thin electric double layer is formed between a positive electrode or a negative electrode and an electrolyte when a voltage is applied. During charging, the electric double layers are formed to store charges, and by discharging, charged particles return to the state before charging. Thus, the electric double layer capacitor does not utilize a chemical reaction. Accordingly, even when the electric double layer capacitor is repeatedly charged and discharged, neither heat generation nor deterioration occurs, and it can be charged and discharged quickly and highly efficiently. Therefore, it is believed that good cycle characteristics can be achieved.
PTD 1 proposes an all-solid electric double layer capacitor including a solid electrolyte and power collectors, wherein the solid electrolyte is an inorganic solid electrolyte.
In PTD 1, the solid electrolyte made of an inorganic compound is used to avoid occurrence of liquid leakage, because using a liquid electrolyte (electrolytic solution) may lead to deterioration due to liquid leakage.
Specifically, in PTD 1, a Li ion conductive compound having a NASICON-type crystal structure represented by Li1.3Al0.3Ti1.7(PO4)3 is used, and the solid electrolyte mainly composed of the Li ion conductive compound is fabricated, with a diameter of 14.5 mm and a thickness of 0.97 mm. Then, electrodes made of Au are formed on both surfaces of the solid electrolyte to obtain the all-solid electric double layer capacitor having an electrostatic capacitance of 20 μF.
PTD 1: Japanese Patent Laying-Open No. 2008-130844 (claim 1, paragraphs [0050] to [0051], Table 1, and the like)